1. Field of the Invention
The present invention relates to an acoustic gas monitor and in particular to a monitor employing the transmission and subsequent detection of acoustic energy through a reference gas of known composition.
2. Description of the Prior Art
It is known, for example from U.S. Pat. No. 5,581,014, to provide a general purpose apparatus for deriving compositional information about a gas mixture that may be used to monitor the presence of a gaseous substance in a host gas, for example the presence of ozone in air or oxygen. This known apparatus has a measurement cell for receiving a gas sample to be measured; a physically separate reference cell for receiving a reference gas of known composition, and means for transmitting and detecting sound waves separately and simultaneously through gas in each of the measurement cell and the reference cell. An analyzer is provided to derive the compositional information about the gas in the measurement cell based on the velocity of sound, as obtained from measured transit times of the sound waves, in each of the measurement and the sample cell.
It is further known, for example from U.S. Pat. No. 5,689,060, to provide a device specifically for monitoring the presence of moisture (the gaseous substance) in air (the host gas). This known device has a reference cell containing datum (reference) air and a measurement cell in to which air having an unknown moisture content is introduced. Acoustic energy is transmitted through air in each of the reference cell and the measurement cell and subsequently detected. An analyzer is provided to derive moisture content information based on the difference in the acoustic velocity between the reference cell and the measurement cell, as obtained from transit time or phase difference measurements. However, changes in the composition of the host gas unrelated to moisture content, for example in the levels of carbon dioxide present in the air, will also cause changes in the measured acoustic velocity and may affect the accuracy of the device.
The above object is achieved in accordance with the principles of the present invention in an acoustic gas monitor having a measurement chamber for receiving a gaseous substance to be monitored, and an acoustic velocity meter for determining an acoustic velocity within the chamber and for providing an output indicative thereof, wherein the measurement chamber has a wall section for selective transmission of the gaseous substance between a reference gas contained in the interior of the chamber and a host gas disposed at the exterior of the chamber.
By providing for the selective transmission of a gaseous substance to be measured between internal and external a measurement chamber that holds a reference gas, of known composition, then the presence and even the amount of the gaseous substance in a host gas external the chamber can be monitored by monitoring the propagation of acoustic energy in only a single chamber.
Moreover, by providing for the selective transmission into and out of the chamber of only the gaseous substance to be measured. The accuracy of the monitor is made insensitive to unrelated changes in the composition of the host gas.
The selective transmission may be achieved by providing a wall section consisting of a selectively permeable material, chosen to permit the transfer of only the gaseous substance to be monitored between the interior and the exterior of the chamber.